Evolution of the Atmosphere: Composition,
Structure and Energy

I inhale great draughts of space,
The east and west are mine, and the north and the south are mine
I am larger, better than I thought,
I did not know I held so much goodness - all seems beautiful to me.

Driving Questions:

What gases in the atmosphere are important to life and how are
they maintained?

What natural variations occur in atmospheric constituents and
what are the important time scales for change?

1. The Earliest Atmosphere, Oceans, and Continents

After loss of the hydrogen, helium and
other hydrogen-containing gases from early Earth due to the Sun's radiation,
primitive Earth was devoid of an atmosphere. The first
atmosphere was formed by outgassing of gases trapped in the interior of
the early Earth, which still goes on today in volcanoes.

For the Early Earth, extreme volcanism occurred during differentiation, when massive heating
and fluid-like motion in the mantle occurred. It is likely that the bulk
of the atmosphere was derived from degassing early in the Earth's history.
The gases emitted by volcanoes today
are in Table 1 and in Figure.

Composition of volcanicgases for three volcanoes

Volcanic outgassing

Oxygen in the Atmosphere

Stromatolite and Banded-iron Formation (BIF)

Life started to have a major impact on the environment once
photosynthetic organisms evolved. These organisms, blue-green algae (picture of
stromatolite, which is the rock formed by these algae), fed off atmospheric
carbon dioxide and converted much of it into marine sediments consisting
of the shells of sea creatures.

While photosynthetic life reduced the carbon dioxide content of the
atmosphere, it also started to produce oxygen. For a long time, the oxygen
produced did not build up in the atmosphere, since it was taken up by rocks, as
recorded in Banded Iron Formations (BIFs; picture) and continental red beds. To this day, the majority of oxygen
produced over time is locked up in the ancient "banded rock" and "red
bed" formations. It was not until probably only 1 billion years ago that
the reservoirs of oxidizable rock became saturated and the free oxygen
stayed in the air.

The oxidation of the the mantle rocks may have played an important role in the rise of oxygen. It has been hypothesized the the change from predominantly submarine to subaerial volcanoes may have also led to a reduction in volcanic emission of reduced gases.

Once oxygen had been produced, ultraviolet light split the
molecules,
producing the
ozone UV shield as a by-product. Only at this point
did life
move out of the oceans and respiration evolved. We will discuss these
issues in greater detail later on in this course.

Early Oceans

The Early atmosphere was probably dominated
at first by water vapor, which, as the temperature dropped, would
rain out and form the oceans. This would have been a deluge of truly global
proportions an resulted in further reduction of CO2. Then the atmosphere
was dominated by nitrogen, but there was certainly no oxygen
in the early atmosphere. The dominance of Banded-Iron Formations (BIFs; see
picture)
before 2.5Ga indicates that Fe occurred in its reduced state (Fe2+). Whereas
reduced Fe is much more soluble than oxidized Fe (Fe3+), it rapidly oxidizes
during transport. However, the dissolved O in early oceans reacted with
Fe to form Fe-oxide in BIFs. As soon as sufficient O entered the atmosphere,
Fe takes the oxidized state and is no longer soluble. The first occurrence
of redbeds, a sediments that contains oxidized iron, marks this major transition
in Earth's atmosphere.

Cumulative history of O2 by photosynthesis over geologic time.
The start of free O is likely earlier than shown.

Early Continents

Lava flowing from
the partially molten interior spread over the surface and solidified to
form a thin crust. This crust would have melted and solidified repeatedly,
with the lighter compounds moving to the surface. This is called differentiation.
Weathering by rainfall
broke up and altered the rocks. The end result
of these processes was a continental land mass, which would have grown over
time. The most popular theory limits the growth of continents to the first
two billion years of the Earth.

2. Evolution of the Present Atmosphere

The evolution of the atmosphere
could be divided into four separate stages:

and the first three steps were discussed in detail. The composition
of the present atmosphere however required the formation of oxygen
to sufficient levels to sustain life, and required life to create
the sufficient levels of oxygen. This era of evolution of the atmosphere
is called the "Biological Era."

The Biological Era - The Formation of Atmospheric
Oxygen

The biological era was marked by the simultaneous decrease in atmospheric
carbon dioxide (CO2) and the increase in oxygen (O2) due to life
processes. We
need to understand how photosynthesis could have led to maintenance
of the ~20% present-day level of O2.
The build up of oxygen had three major consequences that we should
note here.

Firstly, Eukaryotic metabolism could only have begun once the level
of oxygen had built up to about 0.2%, or ~1% of its present abundance.
This must have occurred by ~2 billion years ago, according to the
fossil record. Thus, the eukaryotes came about as a consequence
of the long, steady, but less efficient earlier photosynthesis carried
out by Prokaryotes.

Figure 1. Photolysis of water vapor and carbon
dioxide produce hydroxyl and atomic oxygen, respectively, that, in
turn, produce oxygen in small concentrations. This process produced
oxygen for the early atmosphere before photosynthesis became dominant.

Oxygen increased in stages, first through photolysis
(Figure 1) of water vapor and carbon dioxide by ultraviolet energy
and, possibly, lightning:

H2O -> H + OH

produces a hydroxyl radiacal (OH) and

CO2 -> CO+ O

produces an atomic oxygen (O). The OH is very reactive
and combines with the O

O + OH -> O2 + H

The hydrogen atoms formed in these reactions are light
and some small fraction excape to space allowing the O2 to build
to a very low concentration, probably yielded only about 1% of the
oxygen available today.

Secondly, once sufficient oxygen had accumulated in the stratosphere,
it was acted on by sunlight to form ozone, which allowed colonization
of the land. The first evidence for vascular plant colonization
of the land dates back to ~400 million years ago.

Thirdly, the availability of oxygen enabled a diversification of
metabolic pathways, leading to a great increase in efficiency. The
bulk of the oxygen formed once life began on the planet, principally
through the process of photosynthesis:

6CO2 + 6H2O <--> C6H12O6
+ 6O2

where carbon dioxide and water vapor, in the presence of light,
produce organics and oxygen. The reaction can go either way as in
the case of respiration or decay the organic matter takes up oxygen
to form carbon dioxide and water vapor.

Life started to have a major impact on the environment once photosynthetic
organisms evolved. These organisms fed off atmospheric carbon dioxide
and converted much of it into marine sediments consisting of the innumerable
shells and decomposed remnants of sea creatures.

Cumulative history of O2 by photosynthesis through
geologic time.

While photosynthetic life reduced the carbon dioxide content of the
atmosphere, it also started to produce oxygen. The oxygen did not
build up in the atmosphere for a long time, since it was absorbed
by rocks that could be easily oxidized (rusted). To this day, most
of the oxygen produced over time is locked up in the ancient "banded
rock" and "red bed" rock formations found in ancient
sedimentary rock. It was not until ~1 billion years ago that the reservoirs
of oxidizable rock became saturated and the free oxygen stayed in
the air. The figure illustrates a possible scenario.

We have briefly mentioned the difference between reducing (electron-rich)
and oxidizing (electron hungry) substances. Oxygen is the most important
example of the latter type of substance that led to the term oxidation
for the process of transferring electrons from reducing to oxidizing
materials. This consideration is important for our discussion of atmospheric
evolution, since the oxygen produced by early photosynthesis must
have readily combined with any available reducing substance. It did
not have far to look!

We have been able to outline the steps in the long drawn out process
of producing present-day levels of oxygen in the atmosphere. We refer
here to the geological evidence.

When the oceans first formed, the waters must have dissolved enormous
quantities of reducing iron ions, such as Fe2+. These ferrous
ions were the consequences of millions of years of rock weathering
in an anaerobic (oxygen-free) environment. The first oxygen produced
in the oceans by the early prokaryotic cells would have quickly been
taken up in oxidizing reactions with dissolved iron. This oceanic
oxidization reaction produces Ferric oxide Fe2O3
that would have deposited in ocean floor sediments. The earliest evidence
of this process dates back to the Banded Iron Formations, which reach
a peak occurrence in metamorphosed sedimentary rock at least 3.5 billion
years old. Most of the major economic deposits of iron ore are from
Banded Iron formations. These formations, were created as sediments
in ancient oceans and are found in rocks in the range 2 - 3.5 billion
years old. Very few banded iron formations have been found with more
recent dates, suggesting that the continued production of oxygen had
finally exhausted the capability of the dissolved iron ions reservoir.
At this point another process started to take up the available oxygen.

Red Beds

Once the ocean reservoir had been exhausted, the newly created oxygen
found another large reservoir - reduced minerals available on the barren
land. Oxidization of reduced minerals, such as pyrite FeS2
, exposed on land would transfer oxidized substances to rivers and out
to the oceans via river flow. Deposits of Fe2O3
that are found in alternating layers with other sediments of land origin
are known as Red Beds, and are found to date from 2.0 billion years
ago. The earliest occurrence of red beds is roughly simultaneous with
the disappearance of the banded iron formation, further evidence that
the oceans were cleared of reduced metals before O2 began
to diffuse into the atmosphere.

Finally after another 1.5 billion years or so, the red bed reservoir
became exhausted too (although it is continually being regenerated
through weathering) and oxygen finally started to accumulate in the
atmosphere itself. This signal event initiated eukaryotic cell development,
land colonization, and species diversification. Perhaps this period
rivals differentiation as the most important event in Earth history.

The oxygen built up to today's value only after the colonization
of land by green plants, leading to efficient and ubiquitous photosynthesis.
The current level of 20% seems stable.

The Oxygen Concentration Problem.

Why does present-day oxygen sit at 20%? This is not a trivial question
since significantly lower or higher levels would be damaging to life.
If we had < 15% oxygen, fires would not burn, yet at > 25% oxygen,
even wet organic matter would burn freely.

The Early Ultraviolet Problem

The genetic materials of cells (DNA) is highly susceptible to damage
by ultraviolet light at wavelengths near 0.25 µm. It is estimated
that typical contemporary microorganisms would be killed in a matter
of seconds if exposed to the full intensity of solar radiation at
these wavelength. Today, of course, such organisms are protected by
the atmospheric ozone layer that effectively absorbs light at these
short wavelengths, but what happened in the early Earth prior to the
significant production of atmospheric oxygen? There is no problem
for the original non-photosynthetic microorganisms that could quite
happily have lived in the deep ocean and in muds, well hidden from
sunlight. But for the early photosynthetic prokaryotes, it must have
been a matter of life and death.

It is a classical "chicken and egg" problem. In order to
become photosynthetic, early microorganisms must have had access to
sunlight, yet they must have also had protection against the UV radiation.
The oceans only provide limited protection. Since water does not absorb
very strongly in the ultraviolet a depth of several tens of meters
is needed for full UV protection. Perhaps the organisms used a protective
layer of the dead bodies of their brethren. Perhaps this is the origin
of the stromatolites - algal mats that would have provided adequate
protection for those organisms buried a few millimeters in. Perhaps
the early organisms had a protective UV-absorbing case made up of
disposable DNA - there is some intriguing evidence of unused modern
elaborate repair mechanisms that allow certain cells to repair moderate
UV damage to their DNA. However it was accomplished, we know that
natural selection worked in favor of the photosynthetic microorganisms,
leading to further diversification.

Fluctuations in Oxygen

The history of macroscopic life on Earth is divided into three great
eras: the Paleozoic, Mesozoic and Cenozoic. Each era is then divided
into periods. The latter half of the Paleozoic era, includes the Devonian
period, which ended about 360 million years ago, the Carboniferous
period, which ended about 280 million years ago, and the Permian period,
which ended about 250 million years ago.

According to recently developed geochemical models, oxygen levels
are believed to have climbed to a maximum of 35 percent and then dropped
to a low of 15 percent during a 120-million-year period that ended
in a mass extinction at the end of the Permian. Such a jump in oxygen
would have had dramatic biological consequences by enhancing diffusion-dependent
processes such as respiration, allowing insects such as dragonflies,
centipedes, scorpions and spiders to grow to very large sizes. Fossil
records indicate, for example, that one species of dragonfly had a
wing span of 2 1/2 feet.

Geochemical models indicate that near the close of the Paleozoic
era, during the Permian period, global atmospheric oxygen levels dropped
to about 15 percent, lower that the current atmospheric level of 21
percent. The Permian period is marked by one of the greatest extinctions
of both land and aquatic animals, including the giant dragonflies.
But it is not believed that the drop in oxygen played a significant
role in causing the extinction. Some creatures that became specially
adapted to living in an oxygen-rich environment, such as the large
flying insects and other giant arthropods, however, may have been
unable to survive when the oxygen atmosphere underwent dramatic change.

3. Composition of the Present Atmosphere

Comparison to Other Planets

The overall composition of the earth's atmosphere is summarized below
along with a comparison to the atmospheres on Venus and Mars - our
closest neighbors.

The variations in concentration from the Earth to Mars and Venus
result from the different processes that influenced the development
of each atmosphere. While Venus is too warm and Mars is too cold for
liquid water the Earth is at just such a distance from the Sun that
water was able to form in all three phases, gaseous, liquid and solid.
Through condensation the water vapor in our atmosphere was removed
over time to form the oceans. Additionally, because carbon dioxide
is slightly soluble in water it too was removed slowly from the atmosphere
leaving the relatively scarce but unreactive nitrogen to build up
to the 78% is holds today.

Current Composition

The concentrations of gases in the earth atmosphere is now known
to be (ignoring water vapor, which varies between near zero to a few
percent):

CONSTITUENT

CHEMICAL SYMBOL

MOLE PERCENT

Nitrogen

N2

78.084

Oxygen

O2

20.947

Argon

Ar

0.934

Carbon Dioxide

CO2

0.035

Neon

Ne

0.00182

Helium

He

0.00052

Methane

CH4

0.00017

Krypton

Kr

0.00011

Hydrogen

H2

0.00005

Nitrous Oxide

N2O

0.00003

Xenon

Xe

0.00001

Ozone

O3

trace
to 0.00080

The unit of percentage listed here are for comparison sake. For most
atmospheric studies the concentration is expressed as parts per million
(by volume). That is, in a million units of air how may units would
be that species. Carbon dioxide has a concentration of about 350 ppm
in the atmosphere (i.e. 0.000350 of the atmosphere or 0.0350 percent).

Greenhouse Gases

Click to interactively explore Selective Absorbers.

Radiative Properties

Objects that absorb all radiation incident upon them are called "blackbody"
absorbers. The earth is close to being a black body absorber. Gases,
on the other hand, are selective in their absorption characteristics.
While many gases do not absorb radiation at all some selectively absorb
only at certain wavelengths. Those gases that are "selective
absorbers" of solar energy are the gases we know as "Greenhouse
Gases."

The interactive activity to the right allows you to visualize
how each greenhouse gas selectively absorbs radiation. Wien's Law
states that the wavelength of maximum emission of radiation is inversely
proportional to the object's temperature. Using that law we know that
the wavelength of maximum emission for the Sun is about 0.5 µm
(1 µm = 10-6 m) and the wavelength for maximum emission
by the Earth is about 10 µm. In the activity to the right see
where the greenhouse gases absorb relative to those two important
wavelengths.

Released by the combustion
of fossil fuels (oil, coal, and natural gas), flaring of natural
gas, changes in land use (deforestation, burning and clearing
land for agricultural purposes), and manufacturing of cement

Sinks

Photosynthesis and deposition
to the ocean.

Importance

Accounts for about half of all warming potential
caused by human activity.

Used for many years in refrigerators, automobile
air conditioners, solvents, aerosol propellants and insulation.

Sinks

Degradation occurs in the upper atmosphere at
the expenses of the ozone layer. One CFC molecule can initiate
the destruction of as many as 100,000 ozone molecules.

Importance

The most powerful of greenhouse gases  in
the atmosphere one molecule of CFC has about 20,000 times the
heat trapping power on a molecule of CO2.

4. Summary

We developed a few useful tools for the study of biogeochemical cycles.
These include the concepts of the reservoir, fluxes, and equilibria.

Atmospheric evolution progressed in four stages, leading to the
current situation. The atmosphere has not always been as it is today
- and it will change again in the future. It is closely controlled
by life and, in turn, controls life processes. Complex feedback
mechanisms are at play that we do not yet understand.

Oxygen became a key atmospheric constituent due entirely to life
processes. It built up slowly over time, first oxidizing materials
in the oceans and then on land. The current level (20%) is maintained
by processes not yet understood.

Sometime just before the Cambrian, atmospheric oxygen reached
levels close enough to today's to allow for the rapid evolution
of the higher life forms. For the rest of geologic time, the oxygen
in the atmosphere has been maintained by the photosynthesis of the
green plants of the world, much of it by green algae in the surface
waters of the ocean.

Selective absorbers in our atmosphere keep the surface of the
earth warmer than they would be without an atmosphere.